Point-to-multipoint networks such as passive optical networks (PONs) can provide efficient communications from a central location to a plurality of users without exorbitant power demands. A typical PON includes an optical line termination (OLT) acting as a single point on the network side and a plurality of optical network units (ONUs) and/or optical network terminations (ONTs) acting as user terminations on the customer side. Connection between the OLT and the ONTs/ONUs can be achieved via a single or multiple optical fibers that use passive (e.g., non-electrically powered) optical components to affect transmission and reception.
To allow multiple ONTs and ONUs to use a single transmission path to the OLT, the OLT assigns and communicates a downstream signal providing an allocation period to each ONU and ONT instructing each when it may transmit and the various overhead and/or user data rates that it may employ for such transmission. The physical distance from the OLT to the ONUs and ONTs may vary considerably in a PON. Therefore, a PON will perform a ranging procedure, such as a full digital in-band based activation, to measure the distance of each end user location. The ranging procedure determines the different timing delays for each of the ONUs and ONTs, such that these timing delays may be used by the ONUs and ONTs to ensure that their upstream burst transmissions are received at the OLT in a synchronized manner and at the time corresponding to the time allocation originally provided by the OLT.
For Gigabit PON (GPON) networks, in particular, there are three basic requirements for ranging as outlined in the ITU-T recommendations: (1) that the maximum range of the GPON be at least 20 km (this is the maximum differential between the farthest and the nearest ONU/ONT measured from the OLT); (2) no disruption of service to any ONU/ONT other than the one performing transmission delay measurement; (3) when ranging a new ONU/ONT, the other working ONUs/ONTs must temporarily stop transmission. Assuming these conditions are met, the GPON network may be properly ranged.
Careful determination of the ONU/ONT timing delay is key to proper ranging and proper network operation. The length of each upstream frame (from ONU/ONT to OLT) may be the same as the length for a downstream frame (from the OLT to ONUs/ONTs) for all communication rates. Thus, each upstream frame may contain transmissions bursts from each of the ONUs and ONTs, where an upstream bandwidth (BW) allocation map will indicate to the OLT the arrangement of these transmissions and where these bursts are combined using a multiple access protocol, such as time division multiple access (TDMA). If the ranging procedure is not performed properly, and if the ONU/ONT does not itself time its upstream bursts in a manner commensurate with the corresponding timing delay and assigned bandwidth window, then the OLT may fail to properly recover the upstream burst. This failure of course would affect the upstream burst from the incorrectly timed ONU or ONT, but it could also affect the OLT's ability to properly resolve the upstream bursts from other ONUs/ONTs.